1. Field of the Invention
The present invention relates generally to the field of earth boring drill bits and, more particularly, to a roller cone bi-center bit for economically drilling an enlarged borehole below casing in very soft to medium earth formations.
2. Description of the Prior Art
Various bi-center bits have been used for years in various oil and gas well drilling operations. A bi-center bit is designed to drill and under-ream at the same time. It is commonly used to pass through a smaller diameter hole and then drill a larger diameter hole in one operation. It has been used in several applications, such as for example deepening or re-entry of existing wells, drilling areas where gage problems occur, increasing an annulus for cementing, prevention of drill pipe sticking, and enlarging the diameter of the borehole through the production zone.
One problem with conventional underreaming bits is the high failure rate of complex articulating mechanisms. Also, conventional bi-center bits typically include costly PDC dependent cutting devices. Further, conventional bi-center bits are not well adapted to common maintenance activities, such as replacing, repairing, or adjusting components in the field.
A typical bi-center bit is shown and described in U.S. Pat. No. 6,298,929 to Cobos Rojas. The '929 patent describes a bi-center drill bit and drill bit assembly including a cylindrical body having a first diameter section and a smaller second diameter section, each section provided with a threaded end. The cylindrical body includes a longitudinal internal channel with a side channel terminating in a sidewall nozzle. Attached to the external surface of the first diameter section is a rotary cone drill bit with the rotary cone positioned in a trailing position from a stream of drilling fluid from the sidewall nozzle. Two stabilizers are mounted to the cylindrical body substantially opposite from the drill bit to stabilize operation of the bit assembly.
While this and similar devices may be suitable for the particular purpose for which they designed, they are not as suitable for economically drilling an enlarged borehole below casing in very soft to medium earth formations. The bi-center bit shown and described in the '929 patent includes a rigidly mounted under-reaming cutting element which enlarges the hole to only one, predefined diameter and is not adjustable. Further, the bi-center bit of the '929 patent includes a rotary cone in a trailing position that extends from an arm entirely outside the larger diameter section of a length of pipe extending to the pilot bit, and is therefore vulnerable to breakage. In these respects, the roller cone bi-center bit according to the present invention substantially departs from the conventional concepts and designs of the prior art, and in so doing provides an adjustable apparatus primarily developed for the purpose of economically drilling an enlarged borehole below casing in very soft to medium earth formations.